Railroad substations that deliver a DC voltage on the catenaries generally include a diode bridge. Such a diode bridge includes six diodes for rectifying three phases and twelve diodes for rectifying six phases.
Due to their simplicity, such substations are considered to be the only alternative to create DC voltage to power the trains.
However, such substations do not make it possible to recuperate the braking energy produced during a deceleration of a train, since the trains are generally equipped with a high-performance system for recuperating kinetic energy. Such substations also do not make it possible to correct the cos(φ) at the input of the substation and generate many harmonics on the AC grid at the input of the substation and on the DC grid at the output of the substation.
In order to resolve the aforementioned drawback, it is known to recuperate the braking energy of the rail vehicle using a thyristor rectifier. The rectifier is able to regulate the DC output voltage. The DC voltage is the rectified voltage. The recuperation is done via an inverter that recuperates the excess energy from the DC grid. The inverter sends such an excess back to the three-phase grid. In such a case, the inverter is provided with its own transformer.
However, such substations do not make it possible to keep the delivered voltage level at an acceptable value in vacuum and under a load.
Furthermore, such substations are generally a significant source of pollution for the AC grid.